1. Field of the Invention
This invention relates to digital wireless communication systems, and more particularly to methods for efficiently performing candidate frequency searches in digital wireless communications systems.
2. Description of Related Art
Wireless communication systems facilitate two-way communication between a plurality of subscriber mobile radio stations or xe2x80x9cmobile stationsxe2x80x9d and a fixed network infrastructure. Typically, the mobile stations communicate with the fixed network infrastructure via a plurality of fixed base stations. Exemplary systems include such mobile cellular telephone systems as Time Division Multiple Access (TDMA) systems, Code Division Multiple Access (CDMA) systems, and Frequency Division Multiple Access (FDMA) systems. The objective of these digital wireless communication systems is to provide communication channels on demand between the mobile stations and the base stations in order to connect the mobile station users with the fixed network infrastructure (usually a wired-line system).
Basic CDMA Communication System
Mobile stations typically communicate with base stations using a duplexing scheme that allows for the exchange of information in both directions of connection. In most existing communication systems, transmissions from a base station to a mobile station are referred to as xe2x80x9cforward linkxe2x80x9d transmissions. Transmissions from a mobile station to a base station are referred to as xe2x80x9creverse linkxe2x80x9d transmissions. These CDMA systems are well-known in the art. For example, some such system is described in U.S. Pat. No. 4,901,307, issued on Feb. 13, 1990 to Gilhousen et al. and assigned to the owner of the present invention, which is also hereby incorporated by reference for its teachings of CDMA communication systems. Basic radio system parameters and call processing procedures for exemplary prior art CDMA systems is described in the TIA specification, entitled xe2x80x9cMobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,xe2x80x9d TIA/EIA/IS-95-A, published in May 1995 by the Telecommunications Industry Association, and referred to hereafter as xe2x80x9cIS-95Axe2x80x9d. The update and revision to IS-95A and J-STD-008 (PCS specification analogous to IS-95A) is TIA/EIAJIS-95-B, published in Mar. 1999 by the Telecommunications Industry Association, and referred to hereafter as xe2x80x9cIS-95Bxe2x80x9d. Both the IS-95A and IS-95B specifications are hereby incorporated by reference for their teachings on CDMA communication systems.
As shown in FIG. 1, a typical CDMA communication system comprises at least one mobile station and a plurality of fixed base stations geographically distributed over the system""s service area and controlled by a mobile telecommunications switching office (MTSO) 20. The service area is defined as the geographical area within which a mobile station can remain and yet still communicate (i.e., maintain a valid radio link) with the CDMA communication system. Each base station provides communication services to a fixed area within the service area. The service area is known as the base station""s xe2x80x9ccoverage areaxe2x80x9d. Thus, when a mobile station is within a base station""s coverage area the base station is able to provide communication services to the mobile station. A base station that provides service to the mobile is also known as a xe2x80x9cservingxe2x80x9d base station. The MTSO 20 coordinates all of the switching functions between base stations, mobile stations, and other communications systems (e.g., a Public Service Telephone Network or satellite communication system).
Communication between a base station and a mobile station is established by a negotiation process that is initiated upon call origination. The serving base station begins the negotiation process by assigning a selected one of its available forward traffic channels to the mobile station and thus establishes a forward link with the mobile station. The mobile station then establishes a reverse link with the serving base station. Base stations continuously transmit pilot channels. Mobile stations track base station signals through acquiring and tracking pilot channels. Once communication is established between the serving base station and the mobile station, pilot channels emitted by each base station are used by the mobile station to determine the base station coverage area that the mobile station is within and the quality of the link to the base station. As described in more detail below, these pilot channels aid in a procedure known as a xe2x80x9chandoffxe2x80x9d procedure. One such method of a handoff procedure is described in U.S. Pat. No. 5,848,063, issued on Dec. 8, 1998 to Weaver et al., assigned to the owner of the present invention, which is also hereby incorporated by reference for its teachings on CDMA communication systems, especially with respect to its teachings on handoff procedures.
A handoff occurs when a mobile station moves across a xe2x80x9cboundary linexe2x80x9d from a first serving base station""s coverage area to a second base station""s coverage area. The communication system xe2x80x9chands overxe2x80x9d service from the first serving base station to the second base station, also known as the xe2x80x9ctargetxe2x80x9d base station. A handoff also occurs when a single base station utilizes multiple frequency channels and switches communication between frequency channels. Each pilot channel is identified by a pseudorandom noise (PN) sequence offset and a frequency assignment. Thus, each pilot channel is uniquely identified with a base station that transmits the pilot channel. Pilot channels aid mobile stations in performing candidate frequency searches.
A candidate frequency (CF) search is a process by which a mobile station searches for pilot channels on a frequency other than the serving frequency. A CF search is performed to assist the base station in determining if handing off to a different frequency is necessary. Thus, CF searches apply to a handoff between (i.e., a hard handoff). The pilot CF search measures received input powers and signal strengths of pilots. These measurements help determine the base station coverage area on CF""s that the mobile station is within and the quality of the link to the base station. When the mobile station detects a pilot channel of significant strength (i.e., the mobile station is at or very near the frequency coverage boundary of the serving base station or a different base station), it reports the pilot channel to a base station controller. The base station controller initiates a handoff procedure to switch to a target base station (the base station associated with the pilot channel) where the mobile station active set now contains the new pilot channel. The mobile station active set is defined as a set of base stations in communication with the mobile station. Then, as described below, the MTSO 20 switches the radio link from the serving base station to the target base station.
FIG. 1 depicts a simple CDMA communication system having a service area comprising seven base stations controlled by one MTSO 20. Each base station services a separate coverage area, represented by a hexagon in FIG. 1, and communicates with a specific frequency, frequency one (F1) or frequency two (F2). Typically, F1 and F2 operate either on the Cellular band (800 MHz) or the PCS band (1900 MHz). For example, a first base station 12, located in the middle of a Service Coverage Area One, communicates on a first frequency F1. A mobile station 10 is serviced by the first base station 12 because it is located within the Coverage Area One. When the mobile station 10 moves from the Coverage Area One to a Coverage Area Two, it performs a handoff procedure from the first base station 12, the serving base station, to a second base station 14, the target base station. Thus, the mobile station 10xe2x80x2 (of FIG. 1) is now serviced by the second base station 14. It is critical for the MTSO to determine the appropriate time to initiate a handoff to a different frequency than the serving frequency in order to maintain communication with the mobile station during a call.
In CDMA systems, there are two basic types of handoffs, so-called xe2x80x9chard handoffsxe2x80x9d and xe2x80x9csoft handoffsxe2x80x9d. A xe2x80x9csoft handoffxe2x80x9d is a handoff procedure in which the mobile station commences communications with a target base station without interrupting communications with the serving base station. Because mobile stations typically contain only one radio soft handoffs can only be used between base stations with CDMA Channels having identical frequency assignments. Referring to FIG. 1, a soft handoff procedure is performed when the mobile station 10 travels from a first Coverage Area One to a third Coverage Area Three because the base station 12 and a third base station 16 have identical frequency assignments, F1.
Hard Handoff
A xe2x80x9chard handoffxe2x80x9d is defined as a handoff in which a mobile station commences communication with a target base station after a momentary interruption in communication with a serving base station. A hard handoff is used when the serving base stations and the target base stations have differing CDMA channel frequency assignments. A hard handoff can also occur when a single base station utilizes multiple frequency channels and switches communication between frequency channels. The present invention is concerned with the multiple base station scenario, and thus, the single base station scenario is not discussed in detail herein. During a hard handoff, the radio link is momentarily interrupted because a typical mobile station contains only one radio and therefore can only demodulate one frequency at a time. Thus, switching from the CDMA channels of the serving base station frequency to the CDMA channels of the target base station frequency produces a momentary interruption in the continuity of the radio link with the CDMA communication system. As shown in FIG. 1, the first base station 12 is assigned a first frequency F1 and the second base station 14 is assigned a second frequency F2. A hard handoff is performed when the mobile station 10 travels from Coverage Area One to the Coverage Area Two because the first base station 12 and the second base station 14 operate on different frequencies, F1 and F2, as shown in FIG. 1.
FIGS. 2a-2c depict an ideal hard handoff procedure between geographically adjacent base stations. The adjacent base stations, base station one 12 and base station two 14, are assigned different frequency assignments, respectively, F1 and F2. As shown in FIG. 2a, communication between the base station one 12 and the mobile station 10 is established on a serving frequency (SF) 32. The serving frequency is a CDMA channel that operates on F1. The mobile station 10 performs a Candidate Frequency (CF) search for pilot channels either periodically or when requested from a base station. During the CF search the mobile station 10 determines the respective strengths of the pilot channels. The mobile station 10 detects a candidate frequency 34 corresponding to the base station two 14 which transmits on frequency F2 and its associated pilot channel 40 which also transmits on the frequency F2. When the strength of the pilot channel 40 is deemed to be sufficient, the mobile station 10 reports the pilot strength to the base station one 12. The base station one 12 initiates a hard handoff procedure if certain (base station proprietary) conditions are satisfied.
As shown in FIG. 2b, the mobile station 10 momentarily loses communication with the base station one 12 as it acquires the base station two 14 because the mobile station 10 is typically capable of demodulating only one frequency at a time. As shown in FIG. 2c, the mobile station 10 then communicates with the base station two 14 on a new serving frequency 38 (formerly known as the candidate frequency 34). The serving frequency 38 operates on the CDMA channel F2. Thus, the mobile station 10 has been switched from communicating with the base station one 12 on the old serving frequency 32 (FIG. 2a) which operates on CDMA channel F1, to communicating with the base station two 14 on the new serving frequency 38, which operates on the CDMA channel F2.
Disadvantageously, a hard handoff procedure results in an increased probability of dropping a call (i.e., losing a call) than a soft handoff procedure because of the difficulty of controlling the timing of the handoff when switching from the serving base station to the target base station. A complete loss of communication results when the base station initiates a hard handoff too early based on mobile station reported measurements (i.e., before the mobile station 10 is near the base station boundary line 24 (FIGS. 2a-2c)). Referring to FIG. 2b, assume for a moment that the base station 12 initiates a handoff (from BS 12 to BS 14) with the mobile station 10xe2x80x2 prematurely. As shown in FIG. 2d, the mobile station 10 would thereby suffer a complete loss of communication after the base station 12 initiated such a premature hard handoff attempt.
An important objective of CDMA communications is to reduce the probability of dropping a call during a hard handoff procedure. As the communication system increases its accuracy of detecting the coverage boundaries between base stations, the probability of dropping a call is reduced. Thus, industry technical specifications (e.g., IS-95B) and prior art methods have been developed to increase the accuracy determining the best possible instant to initiate mobile station hard handoffs.
Candidate Frequency Search as defined in IS-95B
IS-95B requires the mobile station to perform a xe2x80x9cCandidate Frequency Searchxe2x80x9d (CF Search) to assist the base station in the hard handoff processes. A Candidate Frequency (CF) is defined as a frequency that is transmitted by a base station that is a member of the CDMA communication system and that is not the serving frequency. Each CF contains an associated set of pilot channels. The CF Search allows base stations to more accurately determine when the mobile station is within good coverage of a particular base station, on a given frequency. This increased accuracy in determining the quality of a signal that the mobile station receives from a particular base station aids the communication system in performing hard handoff procedures.
In accordance with the IS-95B specification, at the initiation of an IS-95B CF Search, the base station sends a xe2x80x9cCandidate Frequency Search Setxe2x80x9d (CF Search Set) to the selected mobile station. The CF Search Set comprises a set of pilot PN offsets on the Candidate Frequency that the base station instructs the mobile station to search. During a CF Search the mobile station momentarily disconnects from its radio link on the Serving Frequency (SF). Thus, communication with the serving base station is momentarily interrupted. After the mobile station disconnects from the SF, it tunes to the CF and measures the received input power and the signal strengths of the pilot channels specified in the CF Search Set. During a final step of the CF Search, the mobile station tunes back to the SF (i.e., the serving base station) and reports the pilot channels that have received signal strengths exceeding a pre-defined threshold.
The IS-95B standard has defined two types of CF searches, Candidate Frequency Single Search and Candidate Frequency Periodic Search. The CF Single Search occurs when the mobile station is instructed to perform only one search of the CF Search Set. A single search can be performed during one or more visits to the CF. The CF Periodic Search occurs when the mobile station is instructed to perform searches of the CF Search Set at a cyclic period specified by the base station. Thus, the CF Periodic Search can be thought of as a set of multiple CF Single Searches performed at specific cyclic periods. Details of an IS-95B CF Single Search call flow are given below in Table 1.
Example of an IS-95B CDMA Candidate Frequency Single Search During a Typical Call
Table 1 shows the functional requirements of a Candidate Frequency Single Search as defined in IS-95B. In this example, a call is already in progress, thus, a forward link and a reverse link are already established between the mobile station (MS) and the base station (BS). As shown in Table I, the base station sends CF search parameters to the mobile station on the forward traffic channel. These parameters include the CF Search Set. The xe2x80x9cFreshness_Intervalxe2x80x9d is defined as the total time period that the base station will allow the mobile station to search for pilot channels before reporting back to the base station. The mobile station then transmits a message back to the base station reporting its search capabilities. After assessing the mobile station""s search capabilities the base station requests a CF Single Search. A CF Single Search may consist of multiple visits by the mobile station to the CF.
Referring to Table 1, the Single Search Procedure begins with the mobile station storing the Serving Frequency parameters in order to return to the SF after tuning to the Candidate Frequencies. The mobile station discontinues operation on the SF and thus interrupts communications on its radio link (i.e., reverse link) with the base station. The base station""s radio link (i.e., forward channel) is similarly interrupted (i.e., the base station transmits a signal, but the mobile station does not receive the signal). Therefore, until the mobile station re-tunes back to the SF the call will be interrupted. The mobile station can schedule CF Single Search in multiple visits to the CF so that an interruption is not perceptible to a user. A user may or may not perceive an interruption depending upon the duration of the interruption. Typically, a user will not perceive an interruption of less than 60 milliseconds. After the mobile station discontinues operation on the SF it tunes to the Candidate Frequency. After the tuning to the CF, the mobile station measures the input power of the CF. If the CF input power is relatively low compared to the SF input power, then the mobile station may abort tuning to the CF. If the mobile station continues the search on the CF, it measures the Ec/Io (i.e., the signal strength) for all or some of the pilots in the CF Search Set. The mobile station tunes back to the SF and restores the SF parameters in order to continue the call. The mobile station repeats the visit to the CF at an MS scheduled interval until all pilots in the CF search set have been measured. The mobile station then sends a report of only the pilot data measurements above a predetermined threshold to the base station. Finally, the base station decides whether to initiate a hard handoff based on the received pilot signal strengths.
One disadvantage of the IS-95B CF Search technique is the degradation of voice quality due to the interruption of the radio link with the base station during the CF tuning. Normally, a user does not perceive this interruption because of its short duration (e.g., 60 milliseconds). However, an increase in the number of pilot channels in the CF Search Set causes the MS to interrupt its radio link with the serving base station for longer periods. Degradation in voice quality thereby occurs. In dense service areas a relatively large number of pilot channels need to be searched. Thus, it is desirable to increase the number of pilot measurements taken during CF Searches while reducing interference to and degradation of voice quality.
Prior Artxe2x80x94Frequency Beacon Method
One prior art attempt at solving the hard handoff call degradation problems uses frequency beacons. This prior art attempt adds a frequency beacon to a base station operating on a different frequency from an adjacent base station. The prior art frequency beacon only transmits pilot signals. For example, referring again to FIG. 1, the base station two 14 operates on the CDMA channel F2 and the adjacent base station one 12 operates on the CDMA channel F1. The prior art attempts include a frequency beacon with the base station two 14. The base station two 14 transmits a beacon channel on CDMA channel F1.
An exemplary prior art system that uses base station beacons is shown as if operating under ideal conditions in FIGS. 3a-3c. As shown in FIG. 3a, the base station two 14 has a frequency beacon 18. The base station two 14 transmits a beacon channel 36 that operates on a CDMA channel F1. The base station two 14 operates its communication channels on a frequency F2, while the adjacent base station 12 operates its communication channels on a frequency F1. As shown in FIG. 3a, the base station one 12 and the mobile station 10 establish forward and reverse links on a serving frequency 32. The serving frequency 32 operates on CDMA channel F1. Upon detection of the beacon channel 36 transmitted by the frequency beacon 18, the communication system detects that the mobile station 10 is near the base station two 14. Under ideal conditions, this detection occurs without the mobile station 10 breaking its radio link with the base station one 12. The radio link is continuous because both the beacon channel 36 and the serving frequency 32 operate on F1. When the strength of the beacon channel 36 is sufficient, the base station one 12 initiates a hard handoff procedure for the mobile station 10.
As shown in FIG. 3b, during a hard handoff procedure, the mobile station 10 momentarily loses communication with both the base station one 12 and the base station two 14 because the mobile station 10 is capable of demodulating only one frequency at a time. As shown in FIG. 3c, after the hard handoff, the mobile station 10 communicates with the base station two 14 on a new serving frequency 38 that operates on the CDMA channel F2. Thus, after the hard handoffs, the mobile station 10 is switched from communicating with the base station one 12 on the old SF 32 (that operates on CDMA channel F1) to communicating with the base station two 14 on the new SF 38 (that operates on CDMA channel F2). Under the ideal conditions depicted the mobile station 10 interrupts radio link communication with the communication system for only a few milliseconds. This relatively small amount of time allows the base station two 14 and the mobile station 10 to establish a radio link without dropping the call. However, in the real world environment, conditions are seldom ideal.
Disadvantageously, when operating under non-ideal conditions as exist in typical mobile system environments, the frequency prior art beacon methods occasionally drops calls. Under typical operating environments base station coverage boundary lines are often very complex. Operating environments such as road configuration, direction of travel, and propagation interference from buildings, mountains, and the like cause base station boundaries to be difficult to identify. Due to these complex base station boundaries, mobile stations can report a good strength of a frequency beacon, but by the time the base station initiates a hard handoff, the mobile station is no longer in good coverage conditions. Disadvantageously, this error results in dropped calls.
The prior art beacon methods described above as operating under ideal conditions is shown in FIGS. 4a-4c as operating under typical real-world conditions. As shown in FIG. 4a, the base station boundary line 24 is very complex due to real world operating conditions. The base station two 14 uses the frequency beacon prior art method described above in reference to FIGS. 3a-3c. As described above, the base station one 12 and the mobile station 10 establish forward and reverse links on a serving frequency 32 that transmit on CDMA channel F1. Upon detection of the beacon channel 36, the communication system determines that the mobile station 10 is near the base station two 14. The system makes this determination without breaking the radio link with the base station one 12. The radio link is maintained because both the beacon channel 36 and the serving frequency 32 operate on the same CDMA frequency channel F1. When the beacon channel 36 is sufficiently strong, the mobile station 10 reports the pilot strength to the base station which initiates a hard handoff procedure.
As shown in FIG. 4b, the mobile station 10 nears the base station boundary line 24, however, because the boundary line 24 is complex in real world environments, the mobile station 20 does not enter base station two""s 14 coverage area. The mobile station 10 therefore completely loses communication with both the base station one 12 and the base station two 14 due to the hard handoff procedure. As shown in FIG. 4c, the mobile station 10 remains within the coverage area of base station one 12, however the call remains dropped due to the erroneous hard handoff procedure.
Another disadvantage of the prior art frequency beacon methods is the increased costs associated with the necessity of equipping multiple base stations with frequency beacons. The prior art requires additional hardware in order to operate a frequency beacon at each base station. Normally, a CDMA communication system services a large geographical area (e.g., San Diego County) which may require one hundred or more base stations. In such a system, most of the base stations would require a frequency beacon because it is highly likely that a given base station will adjoin at least one other base station that operates on a different frequency. Thus, the cost of equipping all of these base stations with frequency beacons can become prohibitive.
Yet another disadvantage of the prior art attempts is the degradation of voice quality in the mobile station caused by the use of multiple frequency beacons. A frequency beacon increases interference in the communication system because it transmits on the same frequency as the serving frequency. This increased interference is exacerbated as the mobile station approaches the boundary line between adjacent base stations. The increased interference results in poor voice quality in the mobile station.
Still another disadvantage of the prior art attempts is non-compliance with IS-95B that requires that a CF Search be performed before performing a hard handoff. As stated above, the IS-95B CF Search requires a momentary loss of the radio link. This momentary loss can result in a degradation of voice quality. Thus, it is desirable to increase the number of pilots measured during a CF Search while reducing interference to voice quality. It is also desirable to initiate a hard handoff procedure with a reduction in the probability of dropping calls. It is desirable to perform such a CF search technique without increasing either the cost or adversely affecting the capacity of the communication system. The present invention provides such a method and apparatus that addresses these needs by providing a CF Search with a strategy for visiting pilot channels which increases the number of pilots measured while reducing voice quality degradation. The present invention also provides a mechanism for determining the strongest pilot channel that is available.
The present invention is a novel method and apparatus for performing candidate frequency searches in a digital wireless communication system. The present invention performs this candidate frequency search without noticeably degrading voice quality. The two primary functions of the candidate frequency (CF) search method and apparatus of the present invention are: (1) to detect the strongest pilot channel on the candidate frequency in the area of the mobile station; and (2) to maintain adequate voice quality during the CF search. The present candidate frequency search method and apparatus preferably achieves these two primary functions using a two-stage process.
A xe2x80x9cStage One Schedulingxe2x80x9d Technique preferably determines the best strategy for visiting candidate frequencies. Once the best strategy is determined, the Stage One information is passed to a xe2x80x9cStage Two Searchxe2x80x9d Procedure. The Stage Two Search Procedure uses this information in visiting candidate frequencies. During Stage Two, search parameters are processed and the information gathered is used in visiting candidate frequencies. Pilot channels on the candidate frequency having relatively high strengths are noted by the system for possible use in performing a handoff procedure.
The present invention reduces the chance of losing or dropping a call through a novel method of detecting candidate frequency pilots. The present invention aids a mobile station in detecting the strong CF pilots in two stages: (1) a schedule for the best strategy to visit pilot channels is determined; and (2) a search procedure to determine the strong pilot channel is implemented. These two stages of the present invention allow an MS to detect the strong CF pilots and maintain adequate voice quality during the CF Search.
The Stage One Scheduling Technique of the present invention determines the best strategy for a mobile station (MS) to visit the Candidate Frequency while reducing the interruption to the forward link. In an exemplary embodiment of the present invention, the best strategy to visit Candidate Frequencies is represented by the data values Tv, Nmpv, and Nv that are output to the Stage Two Search Procedures for visiting candidate frequencies. Tv is defined as the period of successive visits to the candidate frequency. Nmpv is defined as the number of measurements per visit. Nv is defined as the number of visits during the Freshness_Interval (defined above). The data values, Tv, Nmpv, and Nv are calculated from the mobile station""s search capabilities, the pilot measurement accuracy requirements, and the voice quality optimization requirements.
In the exemplary embodiment of the Stage One Scheduling Technique, the base station exchanges parameter information with the mobile station. These parameters are received in the form of data messages in accordance with IS-95B. The mobile station uses the parameter information to determine the best schedule for a candidate frequency search.
In an alternative embodiment of the Stage One Scheduling Technique, the mobile station determines the parameter information. In accordance with this alternative embodiment, the mobile station does not interact with the base station during the Stage One Scheduling Technique.
In yet another alternative embodiment, the base station performs the Stage One Scheduling Technique without interacting with the mobile station. In accordance with this alternative embodiment, the base station calculates the Stage One Scheduling Technique output values for Tv, Nmpv, Nv. Then, the base station provides this information to the mobile station for the Stage Two Scheduling Technique.
The two stages of the present invention cooperate to aid any wireless communication system in performing a hard handoff. The present invention does so without noticeably degrading voice quality.
The details of the preferred and alternative embodiments of the present invention are set forth in the accompanying drawings and the description below. Once the details of the invention are known, numerous additional innovations and changes will become obvious to one skilled in the art.